Fix warnings about pessimizing return moves for C++11 and higher

Summary:
Throughout the libc++ headers, there are a few instances where
_VSTD::move() is used to return a local variable.  Howard commented in
r189039 that these were there "for non-obvious reasons such as to help
things limp along in C++03 language mode".

However, when compiling these headers with warnings on, and in C++11 or
higher mode (like we do in FreeBSD), they cause the following complaints
about pessimizing moves:

    In file included from tests.cpp:26:
    In file included from tests.hpp:29:
    /usr/include/c++/v1/map:1368:12: error: moving a local object in a return statement prevents copy elision [-Werror,-Wpessimizing-move]
        return _VSTD::move(__h);  // explicitly moved for C++03
               ^
    /usr/include/c++/v1/__config:368:15: note: expanded from macro '_VSTD'
    #define _VSTD std::_LIBCPP_NAMESPACE
                  ^

Attempt to fix this by adding a _LIBCPP_EXPLICIT_MOVE() macro to
__config, which gets defined to _VSTD::move for pre-C++11, and to
nothing for C++11 and later.

I am not completely satisfied with the macro name (I also considered
_LIBCPP_COMPAT_MOVE and some other variants), so suggestions are
welcome. :)

Reviewers: mclow.lists, howard.hinnant, EricWF

Subscribers: arthur.j.odwyer, cfe-commits

Differential Revision: http://reviews.llvm.org/D11394

llvm-svn: 245421

libcxx/include/__config

@@ -716,6 +716,12 @@ template <unsigned> struct __static_assert_check {};
 #define _LIBCPP_CONSTEXPR_AFTER_CXX11
 #endif
 
+#ifdef _LIBCPP_HAS_NO_RVALUE_REFERENCES
+#  define _LIBCPP_EXPLICIT_MOVE(x) _VSTD::move(x)
+#else
+#  define _LIBCPP_EXPLICIT_MOVE(x) (x)
+#endif
+
 #ifndef _LIBCPP_HAS_NO_ASAN
 extern "C" void __sanitizer_annotate_contiguous_container(
   const void *, const void *, const void *, const void *);

libcxx/include/__hash_table

@@ -2108,7 +2108,7 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>::__construct_node(const value_type& __v
     __h.get_deleter().__value_constructed = true;
     __h->__hash_ = hash_function()(__h->__value_);
     __h->__next_ = nullptr;
-    return _VSTD::move(__h);  // explicitly moved for C++03
+    return _LIBCPP_EXPLICIT_MOVE(__h);  // explicitly moved for C++03
 }
 
 #endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
@@ -2124,7 +2124,7 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>::__construct_node(const value_type& __v
     __h.get_deleter().__value_constructed = true;
     __h->__hash_ = __hash;
     __h->__next_ = nullptr;
-    return _VSTD::move(__h);  // explicitly moved for C++03
+    return _LIBCPP_EXPLICIT_MOVE(__h);  // explicitly moved for C++03
 }
 
 template <class _Tp, class _Hash, class _Equal, class _Alloc>

libcxx/include/__tree

@@ -1843,7 +1843,7 @@ __tree<_Tp, _Compare, _Allocator>::__construct_node(const value_type& __v)
     __node_holder __h(__node_traits::allocate(__na, 1), _Dp(__na));
     __node_traits::construct(__na, _VSTD::addressof(__h->__value_), __v);
     __h.get_deleter().__value_constructed = true;
-    return _VSTD::move(__h);  // explicitly moved for C++03
+    return _LIBCPP_EXPLICIT_MOVE(__h);  // explicitly moved for C++03
 }
 
 #endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES

libcxx/include/algorithm

@@ -851,7 +851,7 @@ for_each(_InputIterator __first, _InputIterator __last, _Function __f)
 {
     for (; __first != __last; ++__first)
         __f(*__first);
-    return _VSTD::move(__f);  // explicitly moved for (emulated) C++03
+    return _LIBCPP_EXPLICIT_MOVE(__f);  // explicitly moved for (emulated) C++03
 }
 
 // find

libcxx/include/ext/hash_map

@@ -681,7 +681,7 @@ hash_map<_Key, _Tp, _Hash, _Pred, _Alloc>::__construct_node(const key_type& __k)
     __h.get_deleter().__first_constructed = true;
     __node_traits::construct(__na, _VSTD::addressof(__h->__value_.second));
     __h.get_deleter().__second_constructed = true;
-    return _VSTD::move(__h);  // explicitly moved for C++03
+    return _LIBCPP_EXPLICIT_MOVE(__h);  // explicitly moved for C++03
 }
 
 template <class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>

libcxx/include/map

@@ -1523,7 +1523,7 @@ map<_Key, _Tp, _Compare, _Allocator>::__construct_node_with_key(const key_type&
     __h.get_deleter().__first_constructed = true;
     __node_traits::construct(__na, _VSTD::addressof(__h->__value_.__cc.second));
     __h.get_deleter().__second_constructed = true;
-    return _VSTD::move(__h);  // explicitly moved for C++03
+    return _LIBCPP_EXPLICIT_MOVE(__h);  // explicitly moved for C++03
 }
 
 template <class _Key, class _Tp, class _Compare, class _Allocator>

libcxx/include/unordered_map

@@ -1511,7 +1511,7 @@ unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::__construct_node_with_key(const
     __h.get_deleter().__first_constructed = true;
     __node_traits::construct(__na, _VSTD::addressof(__h->__value_.__cc.second));
     __h.get_deleter().__second_constructed = true;
-    return _VSTD::move(__h);  // explicitly moved for C++03
+    return _LIBCPP_EXPLICIT_MOVE(__h);  // explicitly moved for C++03
 }
 
 template <class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>